The STAT proteins have the dual purpose of, first, signal transduction from ligand-activated receptor kinase complexes followed by nuclear translocation and DNA binding to activate transcription (Darnell et al., 1994, Science 264:1415-1421). To function as specific transcriptional activators, STAT proteins by themselves or in combination with other proteins must have the ability to recognize specific DNA sequence elements in the promoters of their target genes. The binding of the STATs to DNA occurs only after tyrosine phosphorylation when the proteins form either homodimers (Shuai et al., 1994, Cell 76:821-828) or heterodimers (Schindler et al., 1992, Science 257:809-815: Zhong et al., 1994, Proc. Natl. Acad. Sci. USA 91:4806-4810; Zhong et al., 1994. Science 264:95-98) that bind DNA either alone or in combination with other proteins (Fu et al., 1990, Proc. Natl. Acad. Sci. USA 87:8555-8559; Schindler et al., 1992, Science 257:809-815). Since a number of mutations in the STAT proteins block phosphorylation and thus dimerization (Shuai et al., Science 261:1744-1746; Improta et al., 1994, Proc. Natl. Acad. Sci. USA 91:4776-4780), and none of the STAT sequences resembles previously well-defined DNA binding domains in other proteins, it has not been possible to quickly and easily define the DNA binding domains of the STATs.
U.S. Ser. No. 07/980,498, filed Nov. 23, 1992 now abandoned, which is a Continuation-In-Part of copending U.S. Ser. No. 07/854,296, filed Mar. 19, 1992 now abandoned, and International Patent Publication No. WO 93/19179 (published 30 Sep. 1993, by James E. Darnell, Jr. et al.) (each of which is hereby incorporated by reference in its entirety) disclosed the existence of receptor recognition factors, now termed signal transducers and activators of transcription (STAT). The nucleotide sequences of cDNA encoding receptor recognition factors having molecular weights of 113 kD (i.e., 113 kD protein, Stat113, or Stat2), 91 kD (i.e., 91 kD protein, Stat91, or Stat1.alpha.) and 84 kD (i.e., 84 kD protein, Stat84, or Stat1.beta.) are reiterated herein in SEQ ID NOS:1, 3, and 5, respectively: the corresponding deduced amino acid sequences of the STAT proteins are shown in SEQ ID NOS:2, 4, and 6, respectively. Stat84 was found to be a truncated form of Stat91. There is 42% amino acid sequence similarity between Stat113 and Stat91/84 in an overlapping 715 amino acid sequence, including four leucine and one valine heptad repeats in the middle helix region, and several tyrosine residues were conserved near the ends of both proteins. The receptor recognition proteins thus possess multiple properties, among them: 1) recognizing and being activated during such recognition by receptors; 2) being translocated to the nucleus by an inhibitable process (e.g., NaF inhibits translocation); and 3) combining with transcription activating proteins or acting themselves as transcription activation proteins, and that all of these properties are possessed by the proteins described herein. In particular, the proteins are activated by binding of interferons to receptors on cells, in particular interferon-.alpha. (all three Stat proteins) and interferon-.gamma. (Stat91).
U.S. application Ser. No. 08/126,595, filed Sep. 24, 1993 now abandoned, which is incorporated herein by reference in its entirety, relates to identification of functional sites of Stat1.alpha., particularly identification of tyrosine-701 as the phosphorylation site, and the presence of a functional SH2 domain in the protein. This application further disclosed a murine Stat1 homolog (the nucleotide sequence is shown in SEQ ID NO:7; the amino acid sequence is shown in SEQ ID NO:8). Stat1 was further found to be active as a homodimer (Stat1.alpha.--Stat1.alpha., Stat1.alpha.-Stat.beta., and Stat.beta.--Stat.beta.) (U.S. application Ser. No. 08/212,184, filed Mar. 11, 1994, which is incorporated herein by reference in its entirety). Additional Stat proteins, Stat3 (nucleotide sequence in SEQ ID NO:9 and amino acid sequence in SEQ ID NO:10) and Stat4 (nucleotide sequence in SEQ ID NO:11 and amino acid sequence in SEQ ID NO:12), were disclosed and characterized in U.S. applications Ser. No. 08/126,588, filed Sep. 24, 1993 now abandoned, and Ser. No. 08/212,185, filed Mar. 11, 1994 co-pending, each of which is incorporated herein by reference in its entirety.